The ex-employees, who make up one-third of the Engine Group's total shareholders, received the details of their offers by post on Friday, following the announcement of the marcoms group's proposed sale to Lake Capital.
But there has been anger that ex-employees are receiving a lower value for their shares than current employees and are not being given the opportunity to retain shares in the new company.
One shareholder, who asked not to be named, said he is representing about 50 ex-employee shareholders and is formulating a response to the Engine Group, calling for the same terms as current employees.
A spokesman for The Engine Group said: "The board of Engine has carefully considered the strategic alternatives open to the group and has concluded that the offer from Lake Capital is in the best interests of all stakeholders - shareholders, employees and clients. The offer document which has only just been mailed to shareholders explains the board’s thinking in some detail."
It was announced last week that Engine is being sold to the private-equity company Lake Capital in a deal that values the company at just under £100 million.
The management and staff will take shareholdings in a new combined group with Lake Capital’s other investments – the global research and insights company ORC International and the Hollywood-based entertainment and content company Trailer Park.
The combined entity will be called Engine Group and will have revenues of about $400 million (£238 million).
Peter Scott, the group chief executive of Engine, will become the vice-chairman and head of global strategy for the new group. Terry Graunke, Lake’s chairman and co-founder, will be the executive chairman.
Debbie Klein, Rick Eiserman and Simon Kooyman will continue in their roles as the chief executive of Engine UK, Trailer Park and ORC respectively. Ashley Martin, Engine’s group chief finance officer, will retain his position in the new entity.